1. Field of the Invention
The invention relates to a lithographic projection apparatus having a radiation system for supplying a projection beam of electromagnetic radiation; a mask table provided with a mask holder for holding a mask; a substrate table provided with a substrate holder for holding a substrate; a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. Description of Related Art
An apparatus of this type can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies that are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
Up to very recently, apparatuses of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine
The lithographic projection equipment most commonly used today operates at an exposure wavelength of 365 nm (so-called i-line apparatus) or 248 nm (so-called DUV apparatus). However, the ever-decreasing design rules in integrated circuitry have created a demand for even smaller exposure wavelengths xcex, since the resolution that can be attained with lithographic equipment scales inversely with xcex. Consequently, much research has been devoted to finding new light sources operating at wavelengths shorter than 248 nm. Currently, attention is being focused on new wavelengths that can be produced by excimer lasers, such as 193 nm, 157 nm and 126 nm, and researchers hope that such lasers can be refined so as to produce sufficient intensity for lithography purposes (so as to guarantee adequate throughput). In this context, it should be noted that currently available i-line equipment generally employs a mercury lamp with a power of the order of about 3-5 kW, whereas DUV apparatus typically uses excimer lasers with a power of the order of about 5-10 W, or even higher. The intensity demands on the new-wavelength excimer lasers are therefore very high.
The assignee of the current patent application recently announced the successful development of the world""s first fully functional, wide-field, production-level lithographic projection apparatus operating at 193 nm; up to that point, only relatively primitive test tools operating at 193 nm had been available. The introduction of this apparatus was preceded by intense research efforts into source development, illuminator design, and lens materials. During this research, an important difference was observed between the new 193-nm machine and existing 248-nm devices, as will now be discussed.
In experiments leading to the invention, the inventors observed that intense radiative fluxes of 193-nm light caused transient changes in the characteristics of refractive materials placed in their paths (for example, quartz or CaF2 lens elements). Moreover, the same effect was observed by the inventors to occur in various optical coatings present on lenses or mirrors located in the optical path. These changes were observed to affect, for example, the transmissivity of the projection system, thus altering the radiation intensity received at the substrate, even if the intensity delivered by the radiation system (excimer laser) was kept constant; consequently, such effects could cause serious exposure errors on the substrate (e.g. under-exposure of a resist layer). To make matters worse, the inventors observed that these transmissivity changes demonstrated a complex temporal dependence.
Typically, an apparatus as described in the opening paragraph will additionally comprise one or more intensity (energy) sensors. For example, at a test position prior to the mask, it is possible to divert a small portion of the radiation in the projection beam out of the main path of the beam and onto an intensity sensor, thus allowing continual monitoring of the intensity produced by the radiation system. Similarly, it is possible to provide the upper surface of the substrate table with an intensity sensor, located outside the perimeter of the substrate; such a sensor can then be used to calibrate the apparatus on a regular basis, by allowing periodic comparisons of the intensity produced by the radiation system and the actual intensity Is received at the substrate. In analogy to the effects described in the previous paragraph, the inventors discovered that the sensitivity of such sensors could demonstrate a significant temporal drift as a result of irradiation with 193-nm radiation, resulting in intrinsic errors in the intensity measured at substrate level. Needless to say, if there is a (variable) intrinsic error in Is as a result of such sensitivity drift, this will result in a miscalibration of the apparatus, with the attendant risk of exposure errors.
In the case of radiation wavelengths at or above 248 nm, the effects described in the previous two paragraphs have hitherto not been observed. However, in the case of machines operating at 193 nm, these effects can be very serious. For example, in investigative experiments, the inventors observed that, in the case of a step-and-scan test apparatus employing a 5W ArF laser (193 nm) and various optical components comprising quartz and/or CaF2 elements (inter alia a fly-eye lens or light mixing rod, lenses near the reticle masking blades, the main projection lens, etc.) the transmission T along the path of the radiation (between the laser and the substrate table) decreased by as much as 5-7% within 2-3 minutes of initiating irradiation, and then slowly relaxed upward once more (within a time of the order of about 5 minutes) when irradiation was interrupted (or set to another level). Moreover, differences in amplitude and temporal behavior were observed for different optical materials and material combinations. Such large transmission changes can cause serious dose errors at substrate level, with the possibility of large numbers of substrate rejects (particularly in IC manufacture).
It is an object of the invention to alleviate these problems.
This and other objects are achieved in an apparatus as specified in the opening paragraph, characterized in that the electromagnetic radiation has a wavelength less than 200 nm, and that the apparatus further comprises means for maintaining the energy dose D2 at substrate level at a substantially constant value, by substantially compensating for irradiation-induced drift in the intensity Is at substrate level.
For the sake of clarity, the following definitions will be adhered to throughout this text:
1. Intensity Is is the energy Es per unit time t received at substrate level (Es=Isxc3x97t). This will generally be a measured or derived value.
2. Dose Ds is the amount of radiative energy transferred by the projection beam at substrate level in a specific time-interval ts (Ds=Isxc3x97ts). Unless otherwise stated, ts will be taken to be the exposure time te, i.e. the length of time for which a single target area (die) on the substrate is (planned to be) exposed to a radiative flux during a given batch of exposures.
In experiments leading to the invention, the inventors fired a pulsed ArF laser beam through optical elements comprising quartz and/or CaF2. It was found that, as the duty cycle, energy and/or frequency of the laser pulses was varied, the radiative intensity I transmitted through the optical elements also varied. Alternatively, if pulses of a constant duty cycle were fired through the elements for an extended period of time (minutes), then the value of I was seen to undergo a gradual decay towards an asymptotic value which was about 5-7% below the starting value Io. This behavior appeared to be a complicated function of many parameters, such as time, the energy, length and frequency of the laser pulses, and the previous xe2x80x9cirradiation historyxe2x80x9d of the optical elements (a sort of hysteresis effect). However, after much analysis, the inventors were able to model this behavior on the basis of a set of equations (see Embodiment 2, for example). Accordingly, it became possible to predict the ratio I/Io that would be observed at a particular point in an irradiation cycle, on the basis of the previous xe2x80x9chistoryxe2x80x9d of that irradiation cycle.
Once such a prediction could be made with relatively good accuracy, the possibility of correcting such transient changes in I/Io became tangible. Since a reliable prediction was now available, the inventors chose a feedforward correction (anticipatory measure) instead of a feedback correction (reactive measure), inter alia because the latter would necessarily incur a greater time penalty than the former.
According to the invention, the inventors have devised several different ways of achieving the correction according to the invention, which can be used individually or in combination. These can be further elucidated as follows:
(a) It is possible to adjust the intensity output of the radiation system, e.g. by altering the amplitude of the pulses produced by a pulsed laser source, or by adjusting the pulse frequency of that source.
(b) It is possible to dispose a variable filter at some point between the radiation system and the substrate (e.g. in the illuminator, or above the mask), and to use this filter to vary the intensity reaching the substrate. Such a filter may, for example, take the form of a partially transmissive optical element, whose transmissivity Tis a function of the angle of incidence xcex8 of incoming radiation; by varying xcex8, it is then possible to vary T
(c) It is possible to adjust the exposure time te. A drift tendency in Is is then counterbalanced by imposing an inverse tendency on te, so as to keep Ds substantially constant.
(d) In the case of a step-and-scan apparatus (as opposed to a conventional waferstepper), there is yet another manner in which to perform the correction according to the invention. Such a step-and-scan apparatus additionally comprises:
a first driving unit for moving the mask table in a given reference direction parallel to the plane of the table;
a second driving unit for moving the substrate table with a speed v (the so-called scanning speed) parallel to the reference direction so as to be synchronous with the motion of the mask table. The corrective method is then characterized in that irradiation-induced drift in Is is counteracted by appropriate variation of the scanning speed v, so as to keep Ds substantially constant.
A great advantage of methods (c) and (d) with respect to method (a), for example, is that methods (c) and (d) generally allow correction of a wider range of fluctuations in Is, without having to disturb the laser from its optimum operating state. In accordance with the invention, the lower the value of Is, the higher the value of te (method (c)) or the lower the value of v (method (d)) which has to be chosen, and vice versa; in this way, although the intensity Is may change at substrate level, the radiative dose Ds at that level will remain substantially constant.
In general, the inventors have found that, in exposing a Si wafer (e.g. a 20-cm wafer) with a plurality of dies (e.g. of the order of about 100-200 dies), a significant change in Is (e.g. of the order of a few percent) can occur between exposure of the first and last die. However, during exposure of any single die, the variation in Is is typically small (e.g. of the order of about 0.1-0.5%) and may be neglected in many cases without causing serious dose errors. In general, this means that it will usually be sufficient to assess Is and take corrective measures (as in methods (a)-(d) above) just before exposure of each die or (small) group of dies, the value of the correction remaining constant during that particular exposure. Nevertheless, if it is necessary or desirable to further limit the effect of irradiation-induced drift during a single exposure, then the invention also allows adjustments to the degree of correction during the course of any given exposure (xe2x80x9cintra-diexe2x80x9d correction).
According to the invention, a distinction can be made between a basic feedforward correction method and a number of possible extensions that can help to further improve the performance of the inventive apparatus. For example:
A basic method can be completely based on a model that describes the transient effects. In such a case, there is no attempt to update correction parameters (e.g. by using intermediate auto-calibrations against a reference) so as to take the actual momentary transmission of the optics into account. This can be referred to as a xe2x80x9cstatic methodxe2x80x9d;
An extension to such a basic method is to use a regular auto-calibration to make adjustments for deviations between the outcome of the said model and the measured (actual) transmission status. This can be referred to as a xe2x80x9cdynamic methodxe2x80x9d;
In a further extension of this dynamic method, the result of the auto-calibration is used to fine tune one or more parameters of the said model. Consequently, slow changes in the behavior of the transient effects during use of the apparatus (e.g. caused by deteriorations in the optical materials) can be automatically corrected by appropriate adjustment of model parameters. This can be referred to as a xe2x80x9cdynamic method with learning effectxe2x80x9d.
Due to the transient transmission variation described above, it will be desirable from time to time to perform a relative calibration of the energy sensors E1 and E2. If such a calibration is performed at zero order, it will have to be done in the absence of a reticle on the mask table. In current machines, this would entail removal of the reticle from the mask table, which is time-consuming and therefore incurs a throughput penalty. A more elegant approach proposed by the inventors is the provision of a small through-hole in the mask table, outside the area of the mask; in this scenario, one only has to move the mask table so that the through-hole is positioned in the projection beam, thus allowing radiation to reach the sensor E2 without traversing a reticle. in this way, it becomes unnecessary to remove the reticle from the mask table in order to perform a zero-order calibration.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
It should be noted that an article by N. Leclerc et al. in J. Non-Crystalline Solids 149 (1992), pp 115-121, reports the occurrence of transient transmission degradation in high-OH fused silica cubes when irradiated with 215-nm radiation. The article does not, however, report similar effects in CaF2, or in optical coatings on optical elements, or in energy sensors, it does not report any specific work on the complex optical systems used in lithographic devices, and does not recognize the potentially grave consequences for dose control and product quality in the use of such equipment for IC manufacture using high-intensity 193-nm radiation. Neither does the article seek to model the observed effects for the purpose of performing a correction, nor suggest a corrective feedforward as here elucidated.
The extensive research performed by the inventors at 193 nm has led them to postulate that similar trouble with transient effects will occur in lithographic projection apparatuses operating at 157 nm or 126 nm.
Although specific reference has been made hereabove to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.